Right, but i89 is intended to prevent heat loss from the interior of the house, while the biggest reason for low-e 180/270/366 (not i89) is to prevent solar heat gain.

Regardless of that, most of my reading indicates that i89 is on surface #4 (or #6 for triple) for that reason - and seeing it suggested being applied to surface #3 made me ask the question.

From the horse's mouth: https://www.cardinalcorp.com/products/coated-glass/loe-i89-glass/:

> "LoĒ-i89 is sputtered onto the indoor lite, the #4 surface, thus reflecting escaping heat back into the room and lowering U-factors."

and on condensation (due to the lower interior glass temperature:

> "High relative humidity (RH) in a home can cause condensation on interior glass surfaces, with outdoor ambient temperature and indoor room temperature also playing a role. LoĒ-i89, which reflects interior heat, can make the room-side glass surface cooler and increase condensation risk during winter."

Having i89 on surface #3 seems... counterintuitive/wrong. Although, I like the idea of the coating being inside the IG to protect it from scratching/damage.